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A new method was developed to produce lithium plasma for plasma immersion ion implantation. Initially, an argon glow discharge with operation pressure ranging from 2 times 10-1 to 1 mbar is generated by negatively polarizing an electrode from -400 to -1500 V. Small pieces of metallic lithium that are 99.9% pure fill the top of a conic crucible, with a depth of 2 cm, in electric contact with the electrode. Argon ions from the plasma are used to bombard this target, where heat is created by the momentum transfer from the impacting ions to the crucible. By controlling the operation pressure and the electrode voltage polarization, it is possible to easily heat the crucible to temperatures above the lithium melting point (180degC), causing its evaporation. Lithium atoms are then ionized, mainly due to collisions, with argon ions moving toward the crucible. Double Langmuir probe measurements indicated variation in the density of the discharge from 4 times 109 cm-3 to 1010 cm-3 after lithium evaporation. Silicon wafer pieces immersed in this mixed plasma were submitted to repetitive negative high-voltage pulses (3 kV/6 mus/2.5 kHz) to accelerate plasma ions. High strain in the treated layers was measured by high-resolution X-ray diffraction. Photoluminescence intensity increased after annealing. X-ray photoelectron spectrometry measurement revealed lithium implantation in silicon with an atomic concentration of 78% on the top surface and a penetration depth of about 75 nm.
Date of Publication: Oct. 2008